Radio receiving system



Aug. 11, 1936. G R EATON ET AL 2,050,291

' RADIO RECEIVING SYSTEM Filed Nov. 20, 929 2 She ts-Sheet 1 a:

* ZZZ/51725715 555272772 H/Zfirmfiong 29 away Aug. 11, 1936. G. R. EATONET AL 2,050,291

RADIO RECEIVING SYSTEM I Filed Nov. 20, 1929 2 Sheets-Shget 2 .Q RE R8 13 1w, .uQ \-W Lu Patented Aug. 11, 1936 UNITED STATES PATENT OFFICERADIO RECEIVING SYSTEM Illinois Application November 20, 1929. SerialNo. 408,478.

1 Claim.

Our invention relates to radio receiving circuits, and more particularlyto a radio signal amplification system in combination with a source ofrectified alternating current, and the object of our invention is toprovide an improved circuit arrangement for amplifying signal modulatedelectric waves.

A feature of our invention is the provision of a radio signal receivingsystem using in its high frequency amplification stages audion tubes ofthe tetrode type having an equi-potential cathode and a, grid screen andderiving its operating power from a common transformer whose secondarywinding furnishes unmodified alternating current for heating thefilaments of the cathodes and, upon rectification, direct currentpotentials for polarizing the grids, the grid screens, the cathodes andfor establishing a space discharge.

A further feature of our invention is the provision of an improvedautomatic volume control device adapted to maintain automatically theoutput strength of the received signals to a certain predetermined valueregardless of the fact whether said signals arrive from a definitetransmitting station, or from various transmitting stations in thecourse of the tuning operation.

Another feature of our invention is the provision of a volume controlresistance associate-d on one side with the antenna, on the other sidewith the primary winding of the first coupling transformer and designed,while regulating the strength of the received signals, to maintain theimpedance of the said primary winding constant.

Another feature of our invention resides in a manual volume controlwhich is operative on the radio amplifier of our circuit in combinationwith an automatic volume control.

A further feature of our invention resides in the specific arrangementfor operating the low frequency amplifier stages of our receiving setfrom a phonograph pick-up device.

Another feature of our invention has to do with the circuit arrangementof the push-pull ampli fier which is used as a final audio amplificationstage.

A still further feature of our invention is the provision of means foroperating the power supply unit of our receiving set so that differentpower supply voltages may be connected thereto.

For a better understanding of our invention reference may be had to thefollowing description in connection with the accompanying drawings inwhich:

Fig. 1 is a diagrammatic representation of the receiving set circuits;

Fig. 2 illustrates diagrammatically an alternating current rectifyingand power supply unit together with a push-pull amplifier and a loudspeaker circuit, all intended to be used in combination with thereceiving set as illustrated in Fig. 1; and

Fig. 3 is similar to Fig. 2 except for minor modifications to bereferred to subsequently.

Referring now more in detail to Fig. 1 it will be noted that we haveillustrated a radio signal receiving system comprising three highfrequency amplifyingstages connected in cascade and designatedrespectively as -l, 2, and 3, one detector stage designated as 4, andone low frequency amplification stage designated as 5. The threethermionic tubes of the high frequency amplifier t, t and t respectivelyare of the tetrode type having a shielded grid and an equi-potentialalternating current indirectly heated cathode. The

detector tube t and the low frequency amplifier tube t differ from thehigh frequency amplifier tubes only in so far as they have no gridscreen. Alternating current for heating the cathodes of the tubes issupplied to the receiving set by the bus bars and 2| originating in thepower supply unit as illustrated in Figs. 2 and 3. From the bus bars 20and 21 parallel circuits designated respectively as 22, 23, 24, 25, and26 branch off to the heating filaments of the tubes. Direct current forthe tube anodes and for the appropriate polarizing of the controlelectrodes, the grid screens and the cathodes of the tubes is suppliedto the set by thebus bars 30 and 3| in amanner to be hereinafterdescribed. I

Each of the three high frequency amplifier stages I, 2, and 3, and thedetector stage 4 is provided with an individual air core couplingtransformer designated respectively as Tr, Tr Tr and T1 The lowfrequency stage 5 is coupled to the detector by the iron core audiotransformer Tr Each of the secondary windings of the transformers Tr,Tr, Tr and T1 have their one side grounded at G, each of these windingsbeing moreover shunted by a corresponding variable condenser designatedC, C C and C respectively, and designed to form resonant circuits withsaid secondary windings. For the purpose of tuning to the variousfrequencies of the incoming signals, the said condensers C, C C and Care multiple or gang controlled.

Coming now to the arrangement by virtue of which direct currents for thespace discharge, and direct current polarity of the'requisite potentialgradients are supplied to the various electron tubes, it will be notedthat the bus bars 30 and 3| of which the first is positive and thesecond is negative and which form the continuation of similarly markedbus bars in the power supply units of Figs. 2 and 3, are bridged betweentheir respective terminals B+ and B by a resistance strip consisting ofresistances 33, 34, 35, 36, and 3'! connected in series.

The fact that an intermediate point 40 betweenthe resistances 34 and 35is grounded, positions the polarity of the supply terminals B+ and B-with respect to ground so that the ground potential is positive relativeto the negative side of the power supply. Resistance 36 on the negativeside of the resistance strip is linked to ground also by a branchcircuit comprising slide contact 39, the audion tube 90' and theresistances 4| and 42. The operation of the audion tube 90 as part of avolume control arrangement will be subsequently described more indetail.

It is obvious that from the resistance strip arrangement as. describedwe may obtain various operating voltages for the tubes, as desired, onthe potentiometer principle, depending upon which intermediate pointsbetween the supply terminals B+ and B are tapped. The manner in whichthis has been accomplished in the case of each individual tube willbecome clear as we trace the travel and transformation of the modulatedsignal frequencies from the antenna to the last amplifier tubeinclusive.

After being picked up by the antenna 8 the modulated high frequencyoscillations are made to pass. through resistance H! which incombination with its slide contact 9 can be made use of as a volumecontrol, as will be subsequently explained more in detail, and throughthe primary of transformer Tr to ground. On the way they set up similaroscillations in the resonant circuit formed of the secondary winding oftransformer Tr and condenser C. From the latter circuit the. highfrequency oscillations are transmitted in the well known manner acrossthe coupling'condenser H and resistance l2 to the grid I3 of the firstamplifier tube t.. The input circuit of the first amplifier tube can betraced from grid I3, through resistance l2, coupling condenser H,secondary of transformer Tr, condenser to the cathode [5.

The biasing circuit of tube It can be traced from grid l3, throughresistances l2, l1, 4|, and 42 to ground, and from ground acrossresistance 58 to the cathode. The last named resistance traversed by theplate current is the biasing resistance. Resistances: 4| and 42 serve toimpart an additional bias to the grid in connection with the volumecontrol arrangement, as will be subsequently explained. Resistance I!serves as a grid leak and resistance l2, like the similarlyconnectedresistances l9 and 48 in the second andthird amplifier stages,serves as a stabilizer by acting to reduce the undesirable regenerativeenergy-transfer from the plate to grid circuitsv due to straycapacities.

The output circuit can be traced from the anode l5 through the primaryof transformer T1 condensers 46 and 45, to the cathode l5. This is thepath of the high frequency'oscillations, the pathof the direct currentfor the space discharge can be traced from the anode 15, through theprimary of transformer Tr resistances 33 and 34 on the positive side ofthe resistance strip to ground, biasing resistance 59,to-the'cathodeil5. The grid screen I4 is connected to tap on thepositive side of the resistance strip, -said taplillbeing chosen so asto impart to the grid screen M a potential intermediate that of thecathode and the anode, with a view of minimum internal capacity of thetube. It will be noted that tap 60 is common to the grid screens of theother two high frequency amplifier tubes t and t and that the three gridscreens i4, 52 and 62 are linked to ground by individual condensers 44,56, and 56 respectively. The function of these condensers is to offer alow impedance path to high frequency currents.

Condenser 43 serves to by-pass any stray frequencies which mightotherwise be impressed upon the grid. Likewise, condenser 46 besidessupplying a low impedance path in the output circuit serves to by-passany stray frequencies which might otherwise, through the internalcapacity of transformer T1 become ingrafted upon the signal frequencies.Condenser 45 offers a. low impedance path for both, the input and outputcircuits.

From the anode 16 of tube 75' the magnified signal oscillations aretransmitted to the grid 5! of tube t through the instrumentality ofcoupling. transformer Tr the resonant circuit formed of its secondaryWinding and condenser C the coupling condenser l8, and resistance [9.The input circuit of tube t can be traced from grid 5|, throughresistance l9, coupling con-denser l8, secondary of transformer Trcondenser 5'! to the cathode 53.

The biasing circuit of tube t can be traced from grid 5!, through theresistance l9, grid leak 21, resistances 42 and 50 to the cathode 53.The initial biasing resistance 50 is common to the first two highfrequency amplifier tubes. Resistance 42 serves to impart an additionalbias to the grid 5| in connection with the volume control arrangement.

The output circuit can be traced from the anode 54 through the primarywinding of transformer Tr by-pass condensers 58 and 57 to the cathode53. The direct current circuit for the space discharge can be tracedfrom the anode 54, through the primary winding of transformer T1'resistances 33, 34 and 50 to the cathode 53. The grid screen 52, as hasbeen indicated, is connected to tap on the positive side of theresistance strip for the purpose of reducing the internal capacity ofthe tube.

Condenser 55 in addition serves to by-pass stray frequencies. Condenser57 supplies a low impedance path for the signal frequencies in both theinput and output circuits. Condenser 58 in addition to serving as a linkin the out- 4 put circuit, keeps stray frequencies from reaching thetransformer Tr 'From the anode 54 of tube t the magnified signaloscillations are transmitted to the grid 6! of tube 25 in the usualmanner across the coupling transformer T1 and the resonant circuitformed of its secondary Winding and condenser C The input circuit oftube t can be traced from the grid 6|, through resistance 48, couplingcondenser 41, secondary of transformer Tr condenser 61 to the cathode63. The initial biasing circuit can be traced-from the grid 5 I throughresistance 48, grid leak 49, resistances 42 and 68 to the cathode 63. 68serves as an initial or basic biasing resistance, While resistance 42serves to impart to the grid an additional bias in connection with thevolume control arrangement. The output circuit can be traced from theanode 84 through the primary winding of transformer T1 by-passcondensers 69 and 6'! to the cathode 63.

positive side of the resistance bridge.

Condensers 65, 61, and 69 serve the same pur poses as the similarlyconnected condensers in the preceding two amplifier stages. 'From thelast high frequency amplifier tube theimagnified signal oscillations'aretransmitted to the grid 1| of the detector tube t in the usual manneracross the coupling transformer T1' and the resonant circuit formed byits secondary winding and condenser C The input circuit of the detectortube can be traced from the grid Tl, through secondary winding oftransformer T1, condenser 14 to the cathode 12. The output circuit canbe traced from the anode 13, through primary winding of transformer Trcontact 91, condenser 96 and condenser 14, to the cathode 12*. Thedirect current circuit for the space discharge can be traced from theanode 13, the primary winding of transformer Tr switch 80, whose purposeshall be indicated hereinafter, positive terminal B+, resistances 33,34, and 16 to the cathode 12. The biasing circuit can be traced from thegrid through secondary of transformer'Tr biasing resistance 16 to thecathode I2. 7

From the anode 13 of the detector tube the audio frequencies aretransmitted to the grid 8| of the low frequency amplifier tube 13 in theusual manner across the audio transformer Tr The input circuit of tube tcan be traced from the grid 8|, through secondary winding of transformerT1 condenser 86, to the cathode 82.' The output circuit can be tracedfrom the anode 83, over conductor 85, continued in Fig. 2, throughprimary of transformer I94, to bus bar 30, common to Figs. 1 and 2, andcondensers 96 and. 86 to the cathode 82. The direct current circuit forthe space discharge can be traced from the anode 83, over conductor 85,common to Figs. 1 and 2, through primary winding of transformer 04, busbar 30, common to Figs. 2 and 1, resistances 33, 34, and 8'! to thecathode 82. The biasing circuit can be traced from grid 8|, throughsecondary winding of transformer T1 biasing resistance 81 to the cathode82.

Coming now to the volume control arrangement, it has been set forth thatresistance 36 on the negative side of the resistance strip is linked toground by a branch circuit comprising the slide contact 39, the internalresistance of volume control tube 90 and the resistances 4| and 42,which, as has been indicated, are included in the additional biasingcircuits of the three high frequency amplifier tubes. The grid 9| of thevolume control tube 98 is linked on one side through resistance 88 tothe negative supply terminal B, on the other side through the smallcondenser 18 to the anode 64 of the last high frequency amplifier tube tFor the purpose of establishing the proper potential between the anodeand cathode required for the space discharge in tube 96, it is to benoted that the anode 93 is connected to ground which is above negativepotential, while the oath-- ode derives its voltage from the negativeside of the resistance strip. This circuit extends from the anode 93,through resistances 4|, 42, 35 and part of resistance 39, to the cathode92.

Since the resistances 4| and 42 form a part both of the biasing circuitsof the high frequency amplifier and of the volume control circuit, itisevident from the manner in'which they are connectedthat an increase ofthe plate currentof the volume control tube 96, would increase thevoltage drop in these resistances and consequently 7 increase alsothenegative bias of the amplifier tubes, which-in turn operates to reducethe amplitude of the signal oscillations. A decrease of the platecurrent of tube 98 would in a similar man- 'ner operate to increase thevolume of the signal. The plate current in our arrangement can be variedby varying the grid. voltage. The grid to cathode circuit can be tracedfrom the grid 9| through resistances 88 and 31, part of resistance 39 tothe cathode. The resistances just referred to are'proportioned in such amanner as to bring the. operating point of the tube 90 to the lower bendof the static characteristic curve, which, as is well known, causesthetube to function as a rectifier. a v

There are two'waysin which'the plate current of tube 90 can be varied;One is by the operator varying the portion of resistance 39 included inthe grid to cathode circuit, and since the resistances3'l and 39 formpart of the resistance strip, it is obvious that moving the slidecontact 39 to the right increases the voltage drop between B- and theslide contact 39, thus increasing the negative bias of the grid 9| andreducing the plate current. Moving the slide contact 39 to the left.does in a similar manner efiect an increase of theplate current of tube98.

As 'to the second method in which the plate current of the volumecontrol tube 90 is automatically varied, this is done in our arrangementthrough the instrumentality of condenser 10 linking the grid 9| to theanode of the last radio frequency tube. the radio frequency potential inthe radio 'frequency amplifier tube would operate through condenser 19to increase the amplitude of the grid potential oscillations of thevolume control tube 90, and since tube 90 is'adjusted to operate as arectifier, the increased potential oscillations of the grid will operatein the well known manner toincrease the plate current of the tube, whichashas .been previously explained, automatically increases the negativebias of the high frequency amplifier tubes and reduces the output volumeof the signal by decreasing the amplifier gain.

The need for an automatic volume control arises most commonly in thecourse of the tuning operation, when the operator has once adjusted thevolume control device to a certain desired volume output from atransmitting station A, and switches to some other station B which iseither located nearer to the receiver, or is more powerful than A, thenthe volume output would increase if were not for the action of theautomaticvolume control device which holds down the volume output tothepredetermined value; The automatic volume control device also,operates to maintain the signal output from a; certain station at thepredetermined volume, if the signals from the station vary in strength.v

We have indicated two different methods for manual control of the volumeoutput of our receiving set, one is effected by resistance 36 and itsslide contact 39,'the other by resistance"! and its slide contact 9associated with the an tenna 8. The first is to be used in preference bythe operator for finer adjustment and for predetermining the automaticvolume control; the other is to be employed when the signals fromsomepowerful or nearby station are to be reduced It is obvious that anincrease in to a moderatevolume. Resistance I has been proportioned insuch a manner that the impedance of the primary winding of transformerTr remains constant irrespective of the location of slide contact'9,thus excluding the possibility of disturbing the simultaneous resonanceof the three gang-controlled resonant circuits.

Condenser 95 linking the plate 93 of the volume control tube, 90 toground at G operates 1n conjunction with condenser 43 of the firstamplifier stage to supply a low impedance path to undesirable energywhich may feed back into the first stage from the second and the thirdamplier stages.

Condenser Q6 linking the positive bus bar 30 tov ground, in addition toits functions as previously indicated, operates to shunt undesirableripples due to the alternating currents of the power supply unit.

Switch 80 is to be used for operating the receiving set, more exactlyits low frequency amplifier stages, from a phonograph pick-up device.For this purpose the phonograph pick-up device is connected between lead84 and. ground at 89, and switch 80 is lifted so that it opens contact91 and closes contacts 98 and 99.

Coming now to the power supply arrangement as illustrateddiagrammatically in Fig. 2 it may be noted that it consists in'the mainof a power transformer I00 whose primary winding can be connected to asource of commercial alternating current: a full wave rectifier tube IOIfed from a secondary winding of said transformer I00; a filter I 02consisting of an inductance H6 and two condensers I I1 and H8; apush-pullamplifier having tubes I03, an input transformer I04and anoutput transformer I05, the latter being incorporated with the loudspeaker LS, whose electrical connections are representeddiagrammatically at the bottom of the drawings.

The secondary windingof transformer I 00 is subdivided into fourindependent coils I06, I01, I08 and I09, respectively. I06 suppliesalwrnating current for the heating filaments of the tubes through thebus bars 20 and 2I, already referred to in a previous section of ourdescription and to the heater of the automatic volume control tube. Aresistance IIO connected between the bus. bars 20 and 2I has itsmid-point grounded at III for the purpose of eliminating ripples due tothe alternating currents and high frequency currents.

Secondary coil I01 supplies the filaments of the push-pull amplifiertubes I03. Secondary coil I08 supplies the filament of the rectifiertube IOI, its center tap II2 forming the positive terminal of therectified voltage while the center tap H3 of coil I09 connected to theplates of the rectifier tube forms the negative terminal. The manner ofoperation of rectifier tube IOI is well known in the art.

The supply of positive voltage to the receiving set can be traced fromthe positive terminal IIZ, over lead II4, through filter I02, lead II5,field coil of loud speaker I20, lead I2I to tap I22, from where it isdelivered on one side through the primary of the input transformer I04to the anode of the audio frequency amplifier tube, and on the otherside through the bus bar 30, already referred to in a previous section,to the resistance strip as shown in Fig. l.

The supply of negative voltage to the receiving set can be traced fromthe negative terminal II3 of the rectifier over leads I23, I24, and I25to tap I30, said tap I30 being, common to the filter I02,

the input circuit of the push-pull amp fi tube and to bus bar 3| leadingto the negative terminal of the resistance strip in Fig. 1.

The manner in which the filter I02 consisting of the inductance II6incorporated in the positive supply line and condensers I I1 and I I8linking each of its ends to the negative supply line, operates to reducethe direct current flowing in the said supply lines to uniformity, iswell known in the art.

The purpose'of the push-pull amplier in our arrangement is two fold,first, to serve as a final audio-amplifying stage, and second, toeliminate undesirable even harmonics which might have become ingraftedupon the signal wave in the course of the previous amplification.

The input circuit of the push-pull amplifier tubes can be traced fromeither of the grids I28 and I29, through half of secondary winding oftransformer I04, lead I32, to tap I30, over lead I25, through condenserI40, secondary coil I07 to the cathodes I 3I For the purpose ofimparting the proper potential gradient to the anodes of the push-pullamplifier tubes with respect to the cathodes, it will be noted that theanodes I26 and I2! communicate, with the positive terminal II2 of therectifier through the primary winding of the output transformer I05,lead II9, lead II5, filter I02 and lead II4. The average potential ofthe cathodes I 3I on the other hand is brought down to the level of thenegative terminal H3 of the rectifier by connecting the center tap ofcoil I01 supplying the cathodes to the said negative terminal II3through resistance I35, tap I36, lead I24, tap I37, and lead I23. Theelements just enumerated traced from cathode to anode form the outputcircuit of the push-pull amplifier tubes, except that condenser I40should be substituted for resistance I 35.

The audio frequency signal-oscillations passing through the primarywinding of the input transformer I04 induce similar oscillations in thesecondary Winding from whence these are communicated to the grids I28and I 29 and magnified in the output circuits as just traced. Themagnified oscillations combine in the well known manner in the primaryWinding of the output transformer I eliminating even harmonics andinducing corresponding oscillations in the secondary winding from wherethe energy is transferred to the voice coil of a dynamic type of loudspeaker, not shown in the figure, vibrating in a field generated byfield coil I20.

The purpose of condenser I40 shunting the biasing resistance I35, hasbeen indicated, namely, to supply a low impedance path in both the inputand output circuits of the push-pull amplifier tubes.

The primary winding of transformer I 00 is provided with three terminalsI4I, I42, and I43, of these MI and I42 are designed to receive thecommonly available alternating current voltage, while I4I and I 43 canbe used in localities where the voltage is slightly higher. Thecondenser bridge between the terminals MI and I42 consisting ofcondensers I44 and I45 intermediately grounded at I50 is designed tooffer a low impedance path to any undesirable stray high frequencieswhich might have become introduced into the alternating current supplylines.

Referring now to Fig. 3 it will be noted it consists essentially of thesame elements as Fig. 2 except that instead of a double wave rectifiertube IOI (Fig. 2), two single wave rectifier tubes IN and H]! are used.Since the voltage delivered by the rectifier in this case isconsiderably higher than in the case of the single tube rectifier,resistance I60 is inserted between the field coil of loud speaker l2l'and tap I22 in order to bring the voltage supplied to the receiving setdown to the desired value. For the same reason resistance 32' issubstituted for strap 32 between the positive terminal 13+ and tap 56 inFig. 1. Resistance I10 in Fig. 3 connected between the bus bars and 3|serves to stabilize the voltage supplied to the anodes of the radiofrequency amplifier tubes. It also increases the current through thefield coil of the loud speaker.

In general the operation of the system consists in tuning in the desiredsignal by rotating the gang condenser and then setting the automaticcontrol device to the desired volume output. If the volume input was tovary now, the automatic control tube increases or decreases the bias onthe radio amplifier tubes to maintain the volume output constant. If thetuning device is now rotated to tune in another station the automaticvolume control mechanism operates to control the radio amplifier tomaintain the volume output constant. The manual volume control 9 is usedif the set is situated near a powerful broadcasting station and it isdesired to further reduce the volume output.

While we have shown certain specific circuit arrangements it is obviousthat changes and modifications may be made without departing from thespirit of our invention, therefor, the invention should only be limitedby the scope of the appended claim.

Having described our invention, what we claim as new and desire tosecure by United States Letters Patent is:

In an amplifier circuit provided with audion tubes and with couplingtransformers included in the input and output circuits of said tubes, avolume control resistance connected intermediately of the antenna andthe primary winding of first coupling transformer and proportioned tomaintain the total impedance of the circuit associated with said primarywinding constant 20 irrespective of the variations of the volume controlresistance, and resistances in, certain of said input circuits forreducing undesirable regenerative energy.

GEORGE R. EATON. HERBERT M. ARMSTRONG.

